The present invention relates to a wide variety of applications, such as producing ion beams obtained by extracting the plasma, or surface treatments such as etching, deposition, chemical or thermochemical treatment, spraying, cleaning, disinfection, or decontamination.
In the technical field of exiting a plasma to electron cyclotron resonance, resonance is obtained when the gyration frequency of an electron in a magnetic field that is static or quasi-static is equal to the frequency of the applied accelerating electric field. Such resonance is obtained for a magnetic field B and an excitation frequency f that are related by the equation: EQU B=2.pi.mf/e
where m and e are the mass and the charge of an electron.
When exiting a plasma, electron cyclotron resonance is effective only if the electron is accelerated sufficiently, i.e. if the electron can rotate for sufficient time in phase with the electric field to acquire the threshold energy required for ionizing the gas. To satisfy this condition, it is necessary firstly for the radius of gyration of the electron to be small enough, and in particular for it to remain in the three-dimensional region that combines the conditions required for resonance, i.e. the simultaneous presence of the electric field and of the magnetic field, and secondly for the gyration frequency f to be large compared with the frequency with which electrons collide with neutral bodies such as atoms and/or molecules. In other words, the best conditions for exciting a plasma to electron cyclotron resonance are obtained when gas pressure is relatively low and simultaneously electric field frequency f is high, i.e. for a magnetic field magnitude B that is high. In practice, in a conventional plasma, conditions favorable to excitation to electron cyclotron resonance are obtained for frequencies f of the order of or greater than 500 MHz and gas pressures of the order of 0.1 Pascals, or from 10.sup.-3 to a few tens of Pascals depending on the nature of the gas and on the excitation frequency. However, microwave frequencies above 10 GHz require very high magnetic flux densities that would appear not to be within reach of present day permanent magnets and simple magnetic structures. At a frequency f equal to 2.45 GHz, the flux density B is 0.0875 Teslas, and it exceeds 0.35 Teslas at 10 GHz.
French patent No. 85 08 836 describes a technique for exciting a plasma to electron cyclotron resonance that requires permanent magnets to be used, each of which creates at least one surface at constant magnetic field and of magnitude corresponding to electron cyclotron resonance. The microwave energy is conveyed to the resonance zone via plasma exciters or antennas each constituted by a metal wire element. Each exciter extends close to and along a magnet, being disposed over a permanent magnet.
The magnetic field of magnitude equal to the value that gives rise to resonance and the microwave electric field are both localized and confined essentially in the space situated between an exciter and the portion of the enclosure wall placed facing a magnet. In the presence of a gaseous medium at low pressure, electrons are accelerated in the resonance zone and they wind around the magnetic field lines that define a plasma-confinement surface. These field lines are in the form of festoons connecting the pole of a magnet to the opposite pole thereof or to the opposite pole of a consecutive magnet. Along its path, the electron dissociates and ionizes the molecules and atoms with which it collides. The plasma formed in this way in the magnetic field festoons subsequently diffuses from the field lines to form a plasma that is practically free of energetic electrons which remain trapped in the festoons.
The major drawback of the prior art described by that patent lies in the fact that the propagation zone of the microwave energy and the resonance zone where the microwave energy is absorbed are superposed. Microwave propagation can therefore take place only with losses, and the magnitude of the microwave electric field and the density of the plasma both decrease progressively along the exciter away from the microwave source. The plasma obtained has non-uniform density along the exciter, so that such a plasma appears to be unsuitable for most industrial applications.
To remedy the drawbacks mentioned above, French patent application No. 91 00 894 proposes placing the applicator in an inter-magnet zone lying between the wall of the reaction enclosure and the magnetic field lines corresponding to electron cyclotron resonance and interconnecting two adjacent poles of opposite polarity. This zone is particularly suitable for propagating microwave energy since it is practically free of plasma. This advantage results from the fact that plasma diffusion normal to the magnetic field lines is considerably reduced when the magnitude of the static magnetic field increases.
Nevertheless, that technique suffers from a drawback since the zones where the microwave electric field is at a maximum and the electron cyclotron resonance zones where the electric field magnitude matches B do not coincide. To produce excitation of the plasma, it is necessary either to increase the magnitude of the microwave electric field, or else to increase the magnitude of the magnetic field to enlarge the resonance zone. If such conditions are satisfied, a dense and uniform plasma can be produced all along the exciter.
In addition to the drawbacks specific to the two techniques described above, mention should also be made of defects which they have in common, relating to the low volume percentage of useful magnetic field produced by the permanent magnets and to the need to place antennas in a position that is accurate relative to the magnetic circuit. It should also be observed that the presence of permanent magnets covering the outside walls of the reactor makes it difficult to install systems for cooling the walls and makes access to the reaction enclosure difficult.